The present invention relates to the field of ink-drop generators used in inkjet printers. It also relates to a print head and a printer using said ink-drop generator.
The principle of the inkjet printer is now well-known and has been described, for example, in the American patent No. U.S. Pat. No. 3,373,437 granted to Richard G. Sweet. In this type of printer an ink-drop generator produces drops of ink that are electrically charged then deflected or not deflected by deflecting electrodes to print or not print a downstream substrate. The technology of deflected continuous jets has been widely used in industrial marking applications where inks incorporate volatile solvents and/or pigments for mediums that are difficult to mark where the atmosphere is rendered difficult due to the presence of dust and a variable temperature. The printed widths are small and the outputs are high. A generator of small drops, such as that described in French patent No. FR 2 653 063 (granted to the present applicant), generally comprises a single inkjet created from a pressurized inkjet cavity that has a jet nozzle on one of its surfaces.
The ink cavity also has an elongated cylindrical transducer on a surface opposite that which comprises the nozzle. Said transducer vibrates at a high frequency according to a longitudinal mode and constantly fragments the jet into regular, identical, equidistant droplets. The assembly consisting of the ink cavity, transducer and nozzle plate is called an ink-drop generator. The ink-drop generator is associated with charge electrodes, deflecting electrodes and possibly an ink collector to constitute a print head. One or more print heads can be mounted on the same printer. One or more ink-drop generators can also be assembled to constitute a single print head. For instance, patent application Ser. No. 2 653 063 referred to above discloses a print head comprising at least two modulation bodies and therefore at least two nozzles equipped with means for adjusting each jet and a single ink-collector module with a single pipe for returning the ink to the common circuit. This type of print head offers the possibility of printing large characters at a higher rate than that provided by a head with only one jet. The detailed embodiment of the invention described below also comprises two modulation bodies, which are also called acoustic-wave generators, shakers, resonators or transducers in documents concerning this technology, but each body actuates several inkjets.
In the description of the prior art contained in European patent EP 0 449 929 B1 it is recalled in col. 1, lines 24-25 and 54-58 that, for chambers comprising several jets, each nozzle is positioned facing either its own acoustic vibration generator or a section of a longitudinal acoustic generator whose measurements extend parallel to the line formed by the jet-nozzle assembly. The acoustic generator is supplied with sufficient power to print a vibration with ink in a direction parallel to the jet. The patent then points out in col. 2, lines 1-8 that this configuration of the vibration generator relative to the nozzle plate is not indispensable provided certain conditions of resonance are met. If the conditions of resonance are complied with a single acoustic generator can stimulate the ink passing through a line of nozzles or part of a line of nozzles that has a length considerably greater parallel to the line of nozzles than the size of the acoustic generator in the same direction, for example 5 to 10 times larger. The condition to be complied with is that the vibrating body vibrates virtually only in a longitudinal mode and at a resonance frequency that differs by xe2x88x9210% of the excitation frequency of the natural resonance vibrations in the ink of the cavity between the end of the body and the nozzle plate, the width of the body being smaller than the length of the series of nozzles or the part of the series of nozzles associated with said body.
In this patent the lateral walls of the ink chamber have a cross-section perpendicular to the line of nozzles disposed in a V shape. The tip of the V is turned towards the line of nozzles. The section of chamber comprising the V-shaped walls may be changed to enable the height of the V to be varied depending on the density of the ink and therefore the speed of the sound in the ink used.
Patent application WO 98 51503 also describes an ink-drop generator for an inkjet printer with the following characteristics: the lateral walls of a cavity containing the ink consist of interior and exterior walls. The resistance component of the acoustic impedance of the external walls is such that the external walls passively dampen the vibrations of the interior walls by dispersing the vibrations. The reactive component of the acoustic impedance of the external walls is such that the external walls actively inhibit the vibrations of the internal walls, said external walls thus ensuring that each inkjet sprays drops of ink at the same predetermined distance from each respective nozzle. This type of configuration is used to prevent the nozzle bearing plate from bending in a direction parallel to the inkjet when the printer is used.
The present applicant has filed European patent application EP 0 532 406 A1 concerning multijet modules and the juxtaposition of several modules positioned side-by-side to obtain a large printing width. Much of the detailed description of the embodiment given below repeats the description of the above-mentioned application, particularly everything that relates to the mechanical fastening of print modules to a module assembly beam.
As in the examples of embodiments in European patent applications EP 449 929 B1 or EP 0 532 406 A1 referred to above, the invention relates to a multijet print head, i.e. a head in which a cavity containing pressurized ink delivers several jets that are divided into drops by a single resonator for said cavity. As in the embodiment described in European patent application EP 0 532 406 A1, the invention also relates to a print head capable of being mounted such that it is aligned with other heads to constitute a print assembly comprising a large number of jets equidistant from one another capable of simultaneously printing a wide band, for example two or more meters.
The multijet cavities of the prior art described, for example, in patent applications WO 98/51503 or EP 0 449 929 B1 referred to above, enable a single resonator to actuate several jets. However, the end jets, i.e. those leaving the first and last nozzles of the cavity, spray irregularly, produce distorted drops or are formed at variable distances when said end jets are too close to the walls of the cavity.
The inventor of the present invention has used digital simulations to improve the quality of the end jets, for example by using a particular contour of the lateral wall at the nozzle plate, i.e. where said lateral wall is secant to the nozzle plate. Another factor that affects the quality of the end jets is the angle formed by the lateral wall of the cavity with the nozzle plate. The angle is preferably 90xc2x0 along the entire contour of the lateral wall.
The relation between the vibrating surface of the resonator and the surface of the nozzle plate should also be taken into consideration. The relation between the surfaces should preferably be approximately 1, for example between 3/4and 4/3. The shape of the transitional surface between a resonator housing and the cavity also plays a role. Finally, the relation of the cavity measurements is also important. Each of the factors mentioned above provides an improvement and the combination of all or some of the factors enables the spray quality of the end jets to be indistinguishable from the quality obtained with the central jets.
It becomes possible to position the end-nozzles very close to the intersection of the lateral wall of the cavity with the axial line joining the nozzles. Under these conditions, even though the distance between consecutive nozzles may be small, it remains possible to create an alignment of several cavities in which all the nozzles are equidistant despite the thickness of the wall separating two consecutive cavities of the same head or two consecutive print heads.
Compared to known embodiments, the present invention also relates to an ink-drop generator suitable for a wide range of inks that does not require the drop generators to be modified and that can be produced in materials capable of withstanding temperatures to which print heads may be exposed in an industrial environment.
To achieve all these aims, the invention relates to an ink-drop generator for an inkjet printer in which an inkjet is sprayed in drops, said generator particularly comprising:
a generator body,
at least one acoustic wave generator with a body elongated in an axial direction to the inkjets, each generator having a vibrating surface perpendicular to the axial direction of the jets, at least one section comprising the vibrating surface of each acoustic generator being housed in a housing of the drop-generator body,
at least one resonance cavity intended to contain ink, the first section only of each cavity possibly being constituted in a main section of said body constituting the main body of the generator and, in this configuration, a second section in a continuation of said main body of the generator connected to be leaktight to the main body of the generator, each cavity having an ink feed and an ink-feed aperture, each cavity being particularly defined by a nozzle plate and a lateral wall secant to the nozzle plate, the intersection of the lateral wall and the nozzle plate defining a first contour line of the lateral wall, the nozzle plate comprising a plurality of nozzles aligned along an axial direction of the nozzles perpendicular to the axial direction of the jets, the axial direction of the jets and the axial line of the nozzles defining a plane of the jets,
a generator characterized in that the lateral wall of each resonance cavity is secant to the nozzle plate perpendicular to said nozzle plate along the entire first contour line of said wall, the first contour line being formed by two equal segments that are parallel to one another and the axial direction of the nozzles, each segment having two ends: a first and a second end, the two first ends of each segment being connected by a first curved line and the two second ends of each segment being connected by a second curved line.
The lateral surface of the cavity therefore consists of two plane walls parallel to one another and, at the axial line of the nozzles, one of the walls containing one of the segments and the other, the other segment, and two curved connecting walls each containing one of the contour curves.
In one embodiment the connecting curved lines of the segment ends are concave towards the inside of the cavity. In general, in order to facilitate manufacture the curved lines are constituted by semicircles the diameter of which is the space between the two segments. Preferably, in order to facilitate a preferred vibration mode in the fluid the largest measurement 1 of the first contour of the cavity lies along the axial line of the nozzles, the distance between the two segments is approximately l/4 and the height of the lateral wall of the cavity is between l/2 and 3l/4, preferably approximately 5l/8. To enable the vibrations produced by the acoustic-wave generator to be transmitted to the ink contained in the cavity it is necessary to connect the acoustic-wave generator housing to the cavity. The connection is achieved by a hollow connector section defined by a lateral connector surface. Said connector surface is intended to connect, for example, a cylindrical shape with a circular base, the diameter of which is the diameter of the acoustic-wave generator, to a cylindrical shape with a more or less flattened rectangular base that is the shape of the lateral surface of the ink cavity. As described above, the space between the two walls of the largest surfaces of the cavity is preferably equal to l/4. The connector surface is preferably obtained as follows: to create the first section of the surface the cylindrical surface with a circular base, the diameter of which is between l/2 and 3l/4 of the acoustic-wave generator, is extended over the section of its periphery that lies between the two planes defined by the largest plane walls of the cavity separated by a distance of l/4 .
Each of the largest walls and/or a continuation of each wall is also hollowed to obtain a hollow the periphery of which is defined by a curved line in the plane of said wall and part of a circle the diameter of which is equal to the diameter of the acoustic-wave generator, said circle being located in a plane perpendicular to the plane wall of the cavity.
The base of the hollow section, which is defined as described above, may be a conical surface, for example, to obtain a progressive junction between the generator housing and the resonance cavity. This junction forms an opening with a more or less rectangular cross-section between the resonator housing and the resonance cavity. The junction of the walls between the resonator housing and the cavity is achieved progressively.